UK attorney general Jeremy Wright was speaking at a hearing on a legal bid to force the government to seek parliamentary approval before formally starting the Brexit process.

Britain's attorney general Jeremy Wright returns to the high court in London, Britain, during a legal challenge to force the British government to seek parliamentary approval before starting the formal process of leaving the European Union.(Reuters)

Britain cannot reverse the decision to leave the European Union once the formal divorce process has begun with the triggering of Article 50 of the EU’s Lisbon Treaty, attorney general Jeremy Wright told the high court on Monday.

“We do not argue that an Article 50 notice can be revoked and we would like the court to proceed on the basis a notification is irrevocable,” said Wright, the government’s top lawyer.

Asked from the bench if that meant there was no such thing as giving conditional notice, Wright said: “We accept that.”

He was speaking at a hearing on a legal bid to force the government to seek parliamentary approval before formally starting the Brexit process.

Prime Minister Theresa May has said she will trigger Article 50 by the end of March next year and that there will be no parliamentary vote beforehand.

The legal challenge questions whether the government can use a historical power known as the royal prerogative, to decide when, how and whether to make this decision.

The case comes amid wider calls for lawmakers to have a vote over the shape of Brexit negotiations, with many politicians arguing the June 23 referendum only revealed that Britons wanted to leave the bloc but not how.